As a technique in silver halide grains for achieving high sharpness, it is known to design silver halide grains so as to shift the thickness in the direction of light penetration from a light scattering length to reduce light scattering due to the silver halide grains which deteriorate sharpness. In this case, it is necessary to design the thickness of the grain which causes light scattering so as to be shifted to thinner. Accordingly, silver halide grains in such a form as octahedron or cube become smaller in size thereof so that a light-intercepting efficiency of the grain is lowered, resulting in reduction in the sensitivity. It is well known that tabular grains are used as a technique for solving this problem.
It is also known to introduce a high iodide containing core within the grain so as to enhance a quantum yield of silver halide grains. There is disclosed tabular grains comprising a high iodide containing core in JP-A 63-92942 (the term, "JP-A" refers to an "published Japanese patent application).
However, it was found that the grains comprising high iodide core suffered from the defect that they were remarkable in pressure desensitization. The pressure desensitization can be improved by decreasing an iodide content of the high iodide core but it leads to lower the sensitivity, so that it cannot be put to practical use. Further, there is a tendency for the tabular grains to be inferior in pressure resistance owing to the form thereof. Accordingly, there has been desired development of a silver halide emulsion with little light scattering, high sensitivity and improved in pressure desensitization.
There is disclosed in JP-A 62-58237 a technique for improvement of fogging by pressure of silver halide grains, in which, during the course of formation of silver halide grains, iodide ions are rapidly added to the reaction mixture to localize a high iodide within the grain. There is also disclosed in JP-A 3-237450 and 4-350850 a method for improving the pressure fogging of the tabular grains in a similar manner to the above-described method. As apparent these disclosure, internally localized dislocation lines, silver iodide or high iodide containing phase results in an improvement in the pressure fogging.
On the other hand, from the viewpoint of preventing recombination of a free electron and hole which has been considered to be one of inefficiency factors relating to the sensitivity of a silver halide emulsion, it has been known in the art that reduction sensitization is effective in enhancing the sensitivity.
As described in Journal of Photographic Science, Vol. 25, page 19-27 (1979) and Photographic Science and Engineering Vol. 23, page 113-117 (1979), an optimally reduction-sensitized nucleus (speck) contributes to the sensitization according to the following reaction upon exposure to light, as mentioned by Mithell and Lowe in Photographishe Korrespondenz Vol. 1, page 20 (1957) and Photographic Science and Engineering Vol.19, page 49-55 (1975). EQU AgX+h.nu..fwdarw.e.sup.- +h.sup.+ ( 1) EQU Ag.sub.2 +h.sup.+ .fwdarw.Ag.sup.+ +Ag (2) EQU Ag.fwdarw.Ag.sup.+ +e.sup.- ( 3)
In the above, h.sup.+ and e.sup.- represent a free electron and hole produced on exposure to light, h .nu. represents a photon and Ag.sub.2 represents a reduction sensitization speck. Assuming that this mechanism is reasonable, the reduction sensitization nucleus is considered to prevent efficiency-lowering due to the recombination of the electron with the hole and therefore contribute to an increase in sensitivity.
According to Photographic Science and Engineering Vol. 16, page 35-42 (1971) and ibid Vol. 23 page 113-117 (1979), however, the reduction sensitization nucleus is able to trap not only hole but also electron so that a sufficient explanation cannot be provided based on the above mechanism alone.
Unlike a sensitivity speck inherent to silver halide grains described so far, it is dificult to predict a role of the reduction sensitization nucleus in a spectral sensitization region of specral-sensitized silver halide grains because of the latent image forming process thereof being complex.
In a silver halide emulsion spectrally sensitized, unlike an inherent sensitivity region, a sensitizing dye itself absorbs light and therefore the primary process of latent image formation is represented by the following (4) in place of (1) afore-described. EQU Dye+h.nu..fwdarw.Dye.sup.+ +e.sup.- ( 4)
Whether a dye hole (Dye.sup.+) and electron (e.sup.-) represented in the right-hand side are transferred or not to the silver halide grain depends largely on properties of the dye. With regard to the dye hole, a sensitization efficiency is considered to be better in the case where the dye hole is not transferred to the inside of the grain.
This subject is discussed in relation with an oxidation potential of the dye in Photographic Science and Engineering Vol. 24, page 138-143 (1980).
As described in Abstracts of International congress of Photographic Science, page 159-162 (1978) and Photographic Science and Engineering Vol. 17, page 235-244 (1973), it is suggested that a sensitizing dye of which hole remains on the surface of the silver halide grain bleaches a fog speck reduction sensitization speck located on the surface. Therefore, it is presumed that, in a most popular surface latent image forming type silver halide emulsion, the surface latent image is bleached, resulting in desensitization.
However, it is still uncertain that the reduction sensitization is to be applied to either of the surface or the inside of silver halide grains, or what kind of dye is to be effectively combined with the silver halide grains.
There have been known reduction sensitization methods, in which the reduction sensitization is applied to the surface of silver halide grains or during the course of forming the silver halide grains, or to seed crystal grains in advance in the case where the silver halide grains are grown up from the seed crystal grains.
In the case where the reduction sensitization is applied to the surface of the grains, a combination thereof with other sensitization such gold or sulfur sensitization results in an undesirable increase in fog so that it is not suitable for practical use. Contrary to that, in the case where the reduction sensitization is performed during the grain growth (in other words, the reduction sensitization is applied to the inside of the grain), there is no disadvatage as above-described.
Such a method is described in JP-A 48-87825 and 57-179835. There is reported, in these disclosures, an enhancement of inherent sensitivity of silver halide. However, they are silent with respect to spectral sensitivity thereof. This is presumed to be due to that surface latent-image is destructed by a dye hole which remains on the surface of silver halide crystal. It is also contemplated that a reduction sensitization speck localized inside the grain does not effectively trap the dye hole on the surface so that the reduction sensitization cannot be effectively achieved.
Accordingly, in order to accomplish an enhancement of sensitivity of surface latent image-forming type silver halide by a combined use of reduction sensitization and sulfur-gold sensitization, there have been known the following problems from viewpoint of an enhancement of spectral sensitivity.
1. In the case when being internally reduction-sensitized, there is no effect thereof on spectral sensitivity. In the case when being surface reduction-sensitized, any effect on the spectral sensitivity has not definitely proved as yet.
2. In the case when being surface reduction-sensitized, combined use thereof with sulfur-gold sensitization is difficult due to being highly fogged.
Relating to the above problems, there have been disclosed techniques for enhancement of sensitivity of a spectral-sensitized silver halide emulsion and improvements in storage stability and pressure resistance in JP-A 2-105139, 2-108038, 2-125247, 2-127636, 2-130545, 2-150837, 2-168247, 2-235047, 4-232945 and 4-32832.
However, it was found that these techniques led to deterioration in low-intensity reciprocity law failure and remarkable desensitization in cases when, after exposure, being allowed to stand over a long period of time under environment of a high temperature and high humidity.